In a BWR, a core is immersed in water inside a reactor vessel to heat the water for generating steam. The core is made up of a plurality of fuel assemblies, each of which is a bundle of parallel, regularly-spaced metal cladding fuel tubes filled with fuel pellets of enriched fissionable material. The water being heated acts as moderator to generate thermal neutrons for the fission reaction. Control of the reaction is afforded by means of neutron-absorbing control rods extending into the core within control rod channels formed by relatively wide gaps between the fuel assemblies and inserted or withdrawn longitudinally to determine the desired total neutron flux. Typically, the control rods have a cruciform crossection, with four arms extending within the gaps between four adjacent assemblies, which are arranged in a square cross section.
One problem with BWR's is that when a control rod is abruptly moved even a short distance in the withdrawal direction to increase the power output, such as occurs when control movement is made in steps, the water which moves into the space previously occupied by the control rod provides greatly increased neutron flux for the fuel rods at the outside of the fuel assemblies which border the control rod channel. This leads to a rapid temperature rise in the interior bulk of their fuel pellets, with attendant possible damage to their cladding by fission gas release which can cause cladding embrittlement and by known radial thermal expansion fuel-cladding interaction phenomena. This effect can be countered by reducing the flow of water through the reactor to lower the neutron flux of the entire reactor prior to withdrawing the control rod. However, such measures result in a significant power production loss.